Gas turbines used for power generation are designed over a wide range of power outputs and with a variety of configurations and operating conditions. They operate using the Brayton cycle which is a single-phase vapor power cycle. Typical systems are open-loop using atmospheric air as the working fluid. In the Brayton cycle air is compressed then fuel is added and combusted then the combined air and combustion gases are expanded through a turbine. A portion of the work produced by the turbine is used for compression of the incoming air with the balance of the turbine work being made available as net output power. The power output of gas turbines may be used for direct thrust as in pure jet engines, used for work and/or thrust as in fan jets and turboprops, or used for work as in electrical power generation.
Microturbines are a recent focus of gas turbine design. They can be described generally as being small scale and using a single stage of compression/expansion. They are usually recuperated designs that use a single shaft connecting the turbine to the compressor and power generator. They operate by drawing air from the atmosphere past the power generator for cooling purposes then compressing the air, passing it through one side of a recuperator, adding fuel, combusting the mixture, expanding the combustion gases through a turbine then passing them through the other side of the recuperator before release to the atmosphere. The present discussion will focus on microturbines that follow this described operation but it will be apparent to one experienced in the art that what is taught can be applied to other turbine designs.
Microturbines, using the single-stage recuperated design, have been packaged in sizes ranging from 30 kWe to 250 kWe and show an O/A efficiency of about 26%. (see e.g. http://www.capstoneturbine.com/prodsol/products/index.asp). By contrast, small industrial turbines using multi-stage axial design are available as small as 1200 kWe. The multi-stage axial design systems will produce about 24.3% O/A efficiency without the use of a recuperator but with output gearing; such systems have little benefit to be gained from using a recuperator (see e.g. http://mysolar.cat.com/cda/files/126912/7/ds20pg.pdf)
In the present patent application, computer simulations, based on thermodynamic principles well known to those of ordinary skill in the art, have been used to analyze several power generating systems useable for electrical generation which employ the Brayton cycle, and to clarify the operating characteristics and limitations of such prior-art Brayton cycle systems. The invention of the present patent application provides novel Brayton cycle systems which are shown to overcome most of the limitations demonstrated by the above analysis. Analysis of these novel Brayton cycle systems shows that their operating characteristics provide ready means to adapt the operation of these systems to varying heat-energy input, and to control the system for optimum efficiency under conditions of varying heat-energy input.
Many turbine systems used for power generation are operated at constant speed. For large turbines constant speed operation may be selected to match the desired speed of the generator which needs to maintain a constant frequency of output power. Using a constant speed also ensures that the compressor operates in a known regime and avoids surge conditions. Microturbines have an additional reason for constant speed operation since they may be operating above a natural vibration frequency of the shaft. Maintaining a constant speed of operation ensures that uncontrolled vibration is never encountered.
Microturbines are small-scale systems and normally operate at high rpm. Using single-stage operation it is found that the compressor will typically absorb over two-thirds of the power produced by the turbine. In these conditions a recuperator is necessary to recover sufficient turbine exhaust energy to make the efficiency acceptable. It should be noted that energy added to the working gas flow is incremental to the internal energy of the working gas and further, that mass is added to the working gas with the addition of fuel. It is an implicit assumption that the input of energy into the system is a control variable through the control of fuel flow. Although large scale turbine systems using multiple compressor stages and intercoolers can achieve overall power efficiencies up to about 40%, single-stage recuperated microturbines will typically achieve about 26% overall power efficiency at full power production with a 900° C. turbine inlet temperature (TiT).
A prior art microturbine arrangement is shown in FIG. 1. Air, used as working fluid (20), is drawn into compressor (1), sometimes across power generator (4) to effect cooling. Working fluid (22) exits compressor (1) at a higher pressure and temperature and is directed to the first side of recuperator (8). High-pressure working fluid (22) is indirectly heated by heat exchange with turbine exhaust (34) in the recuperator (8) and exits the recuperator (8) at a higher temperature as high-pressure working fluid (26) which is directed into combustor (5). Fuel (40) is added to combustor (5) and burned to increase the temperature of the mixture of working fluid (26) and fuel (40) which exits combustor (5) as hot, high-pressure working fluid (32) and is directed into turbine (2). High-pressure, hot working fluid (32) expands through turbine (2) releasing work into shaft (3) and exits turbine (2) as low-pressure, hot turbine exhaust (34). Heat is transferred from low-pressure, hot turbine exhaust (34) to high-pressure working fluid (22) in recuperator (8), the low pressure exhaust 34 being arranged in counter-flow to the high pressure working fluid (22) in the recuperator 8. Work released into shaft (3) serves to supply the parasitic work required by compressor (1) and excess work is used to drive power generator (4).
It is noted that overall power efficiency drops rapidly as energy input is reduced. Operating a microturbine at 50% power will cause the overall efficiency to drop to about 15½%. This results in 83½% of the full energy input capacity being required to maintain half power output. In fact this microturbine requires almost 70% of its full energy input capacity just to be self-sustaining without producing any output power. This is strictly due to the high percentage of energy used to operate the compressor at full power and operating at constant speed which requires a constant parasitic energy requirement. This limits the usefulness of a microturbine for applications in which full power is not always needed or can be tolerated. The small turndown range is revealed in FIG. 11.
A solar application of the Brayton cycle has been proposed and development work implemented by Solhyco (http://www.greth.fr/solhyco/public/solhyco.php#sol4). In this case compressed working fluid (26) leaving recuperator (8) is directed to solar collector (6) before entering combustor (5). This arrangement is shown in FIG. 2 and operates in the same manner as the microturbine shown in FIG. 1 with added solar energy (42) input into working fluid (26) in solar collector (6). For both the microturbine shown in FIG. 1 and the solar microturbine shown in FIG. 2 there is a limit temperature for working fluid (32) entering turbine (2) which represents the maximum temperature allowed due to equipment restrictions.
In an ideal application, the temperature leaving solar collector (6) would be sufficient to be used in the turbine without added energy in combustor (5) and this will be the assumed case for these discussions. The solar microturbine arrangement of FIG. 2 differs from the microturbine arrangement of FIG. 1 only in that addition of solar energy (42) in solar collector (6) does not add mass to the working gas flow. For all practical purposes added solar energy (42) is independent of the temperature of working fluid (26). All of the turndown limitations discussed for the microturbine also applies to the solar microturbine as well.
When operating at maximum solar energy input capacity for the microturbine system with a TiT of 900° C., the compressor will absorb about 68% of the turbine power produced and the microturbine system will operate at an efficiency of 26%. As power output (4) reduces, the parasitic load of compressor (1) remains constant and thus represents a larger portion of the total power produced by turbine (2). The efficiency reduces rapidly since the parasitic compressor load represents an increasing portion of the total turbine power.
The turndown issue for the solar microturbine shown in FIG. 2 is more influential than for the microturbine shown in FIG. 1 since, in practice, it is difficult to design and maintain the maximum energy input through a solar collector. The implicit understanding of microturbine operation is that fuel input is a control variable and thus efficiency is the primary concern of microturbine operation. However in the solar application we find that the energy input is determined by conditions independent of the solar microturbine system. In this case the primary operational concern is the power output as compared to the energy input. FIG. 11 indicates the level of energy input required to produce output power. If the energy input drops by 25% then the power output drops by almost 75%, which calculates to an efficiency of 9%. If the solar collector energy drops by 35% then the Brayton system ceases to operate. In practice, solar collectors vary much more than 35% from morning to night, through seasons, with changes in atmospheric conditions and under passing clouds. The additional problem is that solar input energy greater than the 100% design could result in a TiT in excess of material restrictions.
The large drop in power production and efficiency associated with a drop of solar energy means that a practical application would need to augment operation with fuel input to maintain reasonable power production thus making the system a solar-assisted power converter.
An externally heated application of the Brayton cycle, in which the working fluid is heated by an external fluid heat source, has been proposed many times in the past. An example of such a power generation approach is the BG100 system developed by Talbotts Biomass Energy Systems Limited of Stafford, UK (http://www.talbotts.co.uk/bgen.htm). This external Brayton cycle arrangement is shown in FIG. 3. In this case compressed working fluid (26) leaving recuperator (8) is directed to heater (19) and heated to become working fluid (32) by indirect energy transfer which cools external hot fluid (50) to become external cool fluid (52). As with the microturbine shown in FIG. 1 and the solar microturbine shown in FIG. 2 there is a limit temperature for working fluid (32) entering turbine (2) which represents the maximum temperature allowed due to equipment limitations. In most practical applications of the external Brayton cycle this temperature restriction is more limiting due to the greater temperature of external hot fluid (50) necessary to maintain heat transfer to working fluid (32), the greater amount of material exposed to these limit temperatures, and the cost associated with specialty materials capable of handling such high temperature.
It should be noted in the arrangement shown in FIG. 3 that the temperature of external cool fluid (52) must be higher than the temperature of compressed working fluid (26) entering heater (19) and thus the amount of energy transferred is not independent of the temperatures of operation. The temperature of external hot fluid (50) is limited by material construction of heater (19) and thus the energy input into heater (19) can only be increased by the increased flow of external hot fluid (50). Since the flow of working fluid (26) is fixed by constant speed of compressor (1) then adding energy into the system requires a greater temperature of working fluid (32) but is limited by the temperature of external hot fluid (50) which, in itself, is limited. It is quickly found that increasing energy input into heater (19) results in very little additional energy entering working fluid (26) with most leaving in external cool fluid (52). Of the additional energy which does enter working fluid (26) only a portion is converted to work in turbine (2) with the balance exiting as increased temperature of turbine exhaust (34). This results in an increase in temperature in working fluid (26) entering heater (19) which, in turn, reduces the temperature difference with external cool fluid (52) and reduces the transfer of energy into working fluid (26). The competing effects of increased flow of external hot fluid (50) and increased temperature of external cool fluid (52) creates a limit in the amount of energy that can be transferred in heater (19). The power production by the arrangement shown in FIG. 3 is less than half of the capacity of the original microturbine equipment for the same compression and flow rates and it has an overall efficiency that is a fraction of the microturbine shown in FIG. 1.